1. Field of the Invention
The present invention relates to a method for producing a transparent plastic article, particularly an optical plastic article such as an optical disc as an information recording medium, a lens and a prism by low strain polymerization of a polymerizable liquid monomer.
2. Description of the Prior Arts
A substrate for an optical disc is made of an optically polished glass plate or an injection moldable resin such as a polycarbonate resin (hereinafter referred to as "PC") and polymethyl methacrylate (hereinafter referred to as "PMMA") and, for some kinds of the optical discs, the substrate is made of casting PMMA. The plastic lens is made of not only the injection moldable resin such as PC and PMMA but also a thermosetting resin such as an aryl resin, an epoxy resin and CR-39 (Trade mark of UCC).
The conventional substrates for the optical disc have some drawbacks. Since the optically polished glass plate should be made by cutting a glass plate into a predetermined size piece, optically polishing it and then forming a recording groove, it cannot fulfill the current commercial needs for production cost and production tact.
Although the plastic substrate made by injection molding of the thermoplastic resin has no significant problem in production cost and production tact, it has comparatively poor optical characteristics due to burning during molding, the presence of foreign particles in the raw material and birefringence or warp caused by molding strain or shrinkage. Thereby, it has great disadvantages in recording density and recording/reproducing capability. In addition, when the plastic substrate for the optical disc makes use of reflectance change caused by phase transition of a recording layer, it is required to have heat resistance up to around 200.degree. C. The plastic substrate for a photo-electro-magnetic disc, it is required to have optical uniformity showing small optical strain such as uniform birefringence. Since the thermoplastic resin does not satisfy such requirements, a thermosetting resin has been used. However, the thermosetting resin has a unsatisfactorily low curing rate. Although an ultraviolet curing resin is cured at a high rate, a highly heat resistant one suffers from large curing shrinkage. Therefore, unless the cure shrinkage is reduced, a surface of the article tends to have flow marks caused by reaction shrinkage and mold releasing and/or by the unreacted monomer which flows onto the shrunk surface having been released from a mold surface due to distribution of reaction rates in the mold, or the substrate tends to crack.
In a method for producing an optical disc wherein photopolymerization of a polymer is effected under such condition that grooves or signals are present on a surface of a stamper or a glass mold, strain in the disc substrate increases so that a value of birefringence unsatisfactorily increases.
In the production of the optical articles such as the optical disc and the plastic optical lens or prism, since the raw material can be filtered, the foreign particle in the raw material cause no problem. However, their productivity is still to be improved since it takes several ten hours for polymerization and the unreacted monomers scatter during metallizing or spattering of a recording layer or an anti-reflection layer.